There are various types of known BICS (Built In Current Sensor). Some are described in U.S. Pat. No. 5,963,038, which shows detecting faults in integrated circuits by measurement of current through a conductor in the integrated circuit by means of a sensor situated in the vicinity of the conductor. The sensor can be constructed in various ways so as to measure the field produced by the current through the conductor. Examples disclosed include a Hall sensor, an MR (magnetoresistive) sensor and a GMR (giant magnetoresistive) sensor. This can enable testing of conductors and their connections which cannot be accessed easily by external test equipment, or for detecting faults in individual ones of parallel paths which would pass a resistivity test even if only one path was conducting.
An MR sensor has a resistance that is dependent on an external magnetic field in the plane of the sensor. Different types of MR sensors exist. Sensors based on anisotropic magnetoresistance (AMR), have been used in magnetic recording heads for several years. AMR sensors have a layer of anisotropic magnetic material and the resistance of this layer is influenced by an external magnetic field, which causes the change in the sense current which flows in the layer. A GMR (Giant MagnetoResistive) sensor has a layer of magnetic material in a fixed direction (pinned layer) and a layer of magnetic material of which the magnetisation direction can be influenced by an external magnetic field (free layer), which causes the change in measured resistance. Depending on the type and construction, an MR sensor is more sensitive in one direction and less sensitive in another direction in the plane of the sensor. The MR sensor shown in the above patent is illustrated in FIG. 1. It is constructed to maximise the effect from the current through the conductor on the resistance of the sensor. FIG. 1 shows an MR (magnetoresistive) sensor 502 situated near conductor 104 in a layer of the integrated circuit above or beneath the layer in which the conductor is realised. The MR sensor 502 comprises connection areas 504 and 506 to which through holes 508 and 510 are connected for measuring the actual resistance of the sensor 502.
The current through the conductor 104 causes a circular magnetic field around the conductor 104 through the plane of the sensor 502 and perpendicular to the conductor 104 in the plane of the sensor 502. The MR sensor 502 is sensitive in this direction, so its resistance is measured along the plane of the sensor 502, parallel to the conductor plane to measure the strength of the magnetic field produced by direct current through the conductor 104. A traditional AMR sensor or a GMR (giant magnetoresistive) sensor can be used.
FIG. 2 shows an alternative arrangement of the MR sensor in the integrated circuit known from the above patent. An MR sensor 602 is situated near the bond pad 106 to which the bond wire 108 is connected. The MR sensor 602 has connection areas 604 and 606 for connecting the sensor 602 to respective through holes 608 and 610. The typical attachment of the bond wire 108 to the bond pad 106 is such that near the bond pad 106 the bond wire 108 is to a certain degree perpendicular to the surface of the bond pad 106. A current through the bond wire 108 then causes a magnetic field 612 in the plane of the bond pad 106 and the MR sensor 602, thus causing a change of the resistance of the MR sensor 602. A second MR sensor 614 may be situated at another side of the bond pad and combined with the MR sensor 602 in order to obtain an arrangement that is more sensitive to the magnetic field 612 than a single MR sensor. In FIG. 2, the MR sensors are positioned perpendicular to the magnetic field 612. They may be positioned at different angles to the magnetic field according to which position is more sensitive. In any case, the resistance of the sensor layer is still measured along the plane of the layer.
At a connection layer of the circuit, the through holes e.g. 508-510 and 608-610, are connected to connection tracks connecting the respective MR sensor 502, 602 to an internal detection circuit or to external measurement points. The resistance of the MR sensor 502, 602 can then be measured inside in the integrated circuit with the detection circuit or outside the integrated circuit with a suitable measurement arrangement.
Such sensors are useful for sensing high currents, but are not sufficiently sensitive for applications such as Quiescent Current (IDDQ) testing. IDDQ testing has shown very good coverage of physical defects such as gate oxide shorts, floating gates, and bridging faults which are not very well modelled by classical fault models, or undetectable by conventional logic tests. The demand for high quality and cost effectiveness has led to widespread use of IDDQ testing as a supplementary test to voltage tests. When combined with other test techniques, it has the potential for eliminating the need for burn-in test. However MOSFET leakage currents are rising rapidly with each technology node, narrowing the difference between the IDDQ levels of a faulty and fault-free circuit.